Fix wrong behavior of DDPStrategy option with simple GAN training using DDP

examples/pytorch/domain_templates/generative_adversarial_net.py

@@ -36,6 +36,14 @@
     import torchvision
 
 
+def _block(in_feat: int, out_feat: int, normalize: bool = True) -> list:
+    layers = [nn.Linear(in_feat, out_feat)]
+    if normalize:
+        layers.append(nn.BatchNorm1d(out_feat, 0.8))
+    layers.append(nn.LeakyReLU(0.2, inplace=True))
+    return layers
+
+
 class Generator(nn.Module):
     """
     >>> Generator(img_shape=(1, 8, 8))  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
@@ -47,19 +55,11 @@ class Generator(nn.Module):
     def __init__(self, latent_dim: int = 100, img_shape: tuple = (1, 28, 28)):
         super().__init__()
         self.img_shape = img_shape
-
-        def block(in_feat, out_feat, normalize=True):
-            layers = [nn.Linear(in_feat, out_feat)]
-            if normalize:
-                layers.append(nn.BatchNorm1d(out_feat, 0.8))
-            layers.append(nn.LeakyReLU(0.2, inplace=True))
-            return layers
-
         self.model = nn.Sequential(
-            *block(latent_dim, 128, normalize=False),
-            *block(128, 256),
-            *block(256, 512),
-            *block(512, 1024),
+            *_block(latent_dim, 128, normalize=False),
+            *_block(128, 256),
+            *_block(256, 512),
+            *_block(512, 1024),
             nn.Linear(1024, int(math.prod(img_shape))),
             nn.Tanh(),
         )

examples/pytorch/domain_templates/generative_adversarial_net_ddp.py

@@ -0,0 +1,263 @@
+# Copyright The Lightning AI team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""To run this template just do: python generative_adversarial_net.py.
+
+After a few epochs, launch TensorBoard to see the images being generated at every batch:
+
+tensorboard --logdir default
+
+"""
+
+import math
+
+# ! TESTING
+import os
+import sys
+from argparse import ArgumentParser, Namespace
+
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+
+sys.path.append(os.path.join(os.getcwd(), "src"))
+# ! TESTING
+
+from lightning.pytorch import cli_lightning_logo
+from lightning.pytorch.core import LightningModule
+from lightning.pytorch.demos.mnist_datamodule import MNISTDataModule
+from lightning.pytorch.strategies.ddp import MultiModelDDPStrategy
+from lightning.pytorch.trainer import Trainer
+from lightning.pytorch.utilities.imports import _TORCHVISION_AVAILABLE
+
+if _TORCHVISION_AVAILABLE:
+    import torchvision
+
+
+def _block(in_feat: int, out_feat: int, normalize: bool = True):
+    layers = [nn.Linear(in_feat, out_feat)]
+    if normalize:
+        layers.append(nn.BatchNorm1d(out_feat, 0.8))
+    layers.append(nn.LeakyReLU(0.2, inplace=True))
+    return layers
+
+
+class Generator(nn.Module):
+    """
+    >>> Generator(img_shape=(1, 8, 8))  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
+    Generator(
+      (model): Sequential(...)
+    )
+    """
+
+    def __init__(self, latent_dim: int = 100, img_shape: tuple = (1, 28, 28)):
+        super().__init__()
+        self.img_shape = img_shape
+
+        self.model = nn.Sequential(
+            *_block(latent_dim, 128, normalize=False),
+            *_block(128, 256),
+            *_block(256, 512),
+            *_block(512, 1024),
+            nn.Linear(1024, int(math.prod(img_shape))),
+            nn.Tanh(),
+        )
+
+    def forward(self, z):
+        img = self.model(z)
+        return img.view(img.size(0), *self.img_shape)
+
+
+class Discriminator(nn.Module):
+    """
+    >>> Discriminator(img_shape=(1, 28, 28))  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
+    Discriminator(
+      (model): Sequential(...)
+    )
+    """
+
+    def __init__(self, img_shape):
+        super().__init__()
+
+        self.model = nn.Sequential(
+            nn.Linear(int(math.prod(img_shape)), 512),
+            nn.LeakyReLU(0.2, inplace=True),
+            nn.Linear(512, 256),
+            nn.LeakyReLU(0.2, inplace=True),
+            nn.Linear(256, 1),
+        )
+
+    def forward(self, img):
+        img_flat = img.view(img.size(0), -1)
+        return self.model(img_flat)
+
+
+class GAN(LightningModule):
+    """
+    >>> GAN(img_shape=(1, 8, 8))  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
+    GAN(
+      (generator): Generator(
+        (model): Sequential(...)
+      )
+      (discriminator): Discriminator(
+        (model): Sequential(...)
+      )
+    )
+    """
+
+    def __init__(
+        self,
+        img_shape: tuple = (1, 28, 28),
+        lr: float = 0.0002,
+        b1: float = 0.5,
+        b2: float = 0.999,
+        latent_dim: int = 100,
+    ):
+        super().__init__()
+        self.save_hyperparameters()
+        self.automatic_optimization = False
+
+        # networks
+        self.generator = Generator(latent_dim=self.hparams.latent_dim, img_shape=img_shape)
+        self.discriminator = Discriminator(img_shape=img_shape)
+
+        self.validation_z = torch.randn(8, self.hparams.latent_dim)
+
+        self.example_input_array = torch.zeros(2, self.hparams.latent_dim)
+
+        # ! TESTING
+        self.save_path = "pl_test_multi_gpu"
+        os.makedirs(self.save_path, exist_ok=True)
+
+    def forward(self, z):
+        return self.generator(z)
+
+    @staticmethod
+    def adversarial_loss(y_hat, y):
+        return F.binary_cross_entropy_with_logits(y_hat, y)
+
+    def training_step(self, batch):
+        imgs, _ = batch
+
+        opt_g, opt_d = self.optimizers()
+
+        # sample noise
+        z = torch.randn(imgs.shape[0], self.hparams.latent_dim)
+        z = z.type_as(imgs)
+
+        # Train generator
+        # ground truth result (ie: all fake)
+        # put on GPU because we created this tensor inside training_loop
+        valid = torch.ones(imgs.size(0), 1)
+        valid = valid.type_as(imgs)
+
+        self.toggle_optimizer(opt_g)
+        # adversarial loss is binary cross-entropy
+        g_loss = self.adversarial_loss(self.discriminator(self(z)), valid)
+        opt_g.zero_grad()
+        self.manual_backward(g_loss)
+        opt_g.step()
+        self.untoggle_optimizer(opt_g)
+
+        # Train discriminator
+        # Measure discriminator's ability to classify real from generated samples
+        # how well can it label as real?
+        valid = torch.ones(imgs.size(0), 1)
+        valid = valid.type_as(imgs)
+
+        self.toggle_optimizer(opt_d)
+        real_loss = self.adversarial_loss(self.discriminator(imgs), valid)
+
+        # how well can it label as fake?
+        fake = torch.zeros(imgs.size(0), 1)
+        fake = fake.type_as(imgs)
+
+        fake_loss = self.adversarial_loss(self.discriminator(self(z).detach()), fake)
+
+        # discriminator loss is the average of these
+        d_loss = (real_loss + fake_loss) / 2
+
+        opt_d.zero_grad()
+        self.manual_backward(d_loss)
+        opt_d.step()
+        self.untoggle_optimizer(opt_d)
+
+        self.log_dict({"d_loss": d_loss, "g_loss": g_loss})
+
+    def configure_optimizers(self):
+        lr = self.hparams.lr
+        b1 = self.hparams.b1
+        b2 = self.hparams.b2
+
+        opt_g = torch.optim.Adam(self.generator.parameters(), lr=lr, betas=(b1, b2))
+        opt_d = torch.optim.Adam(self.discriminator.parameters(), lr=lr, betas=(b1, b2))
+        return opt_g, opt_d
+
+    # ! TESTING
+    def on_train_epoch_start(self):
+        if self.trainer.is_global_zero:
+            print("GEN: ", self.generator.module.model[0].bias[:10])
+            print("DISC: ", self.discriminator.module.model[0].bias[:10])
+
+    # ! TESTING
+    def validation_step(self, batch, batch_idx):
+        pass
+
+    # ! TESTING
+    @torch.no_grad()
+    def on_validation_epoch_end(self):
+        if not self.current_epoch % 5:
+            return
+        self.generator.eval(), self.discriminator.eval()
+
+        z = self.validation_z.type_as(self.generator.module.model[0].weight)
+        sample_imgs = self(z)
+
+        if self.trainer.is_global_zero:
+            grid = torchvision.utils.make_grid(sample_imgs)
+            torchvision.utils.save_image(grid, os.path.join(self.save_path, f"epoch_{self.current_epoch}.png"))
+
+        self.generator.train(), self.discriminator.train()
+
+
+def main(args: Namespace) -> None:
+    model = GAN(lr=args.lr, b1=args.b1, b2=args.b2, latent_dim=args.latent_dim)
+
+    # ! `MultiModelDDPStrategy` is critical for multi-gpu training
+    # ! Otherwise, it will not work with multiple models.
+    # ! There are two ways to run training codes with previous `DDPStrategy`;
+    # ! 1) activate `find_unused_parameters=True`, 2) change from self.manual_backward(loss) to loss.backward()
+    # ! Neither of them is desirable.
+    dm = MNISTDataModule()
+    trainer = Trainer(
+        accelerator="auto",
+        devices=[0, 1, 2, 3],
+        strategy=MultiModelDDPStrategy(),
+        max_epochs=100,
+    )
+
+    trainer.fit(model, dm)
+
+
+if __name__ == "__main__":
+    cli_lightning_logo()
+    parser = ArgumentParser()
+
+    # Hyperparameters
+    parser.add_argument("--lr", type=float, default=0.0002, help="adam: learning rate")
+    parser.add_argument("--b1", type=float, default=0.5, help="adam: decay of first order momentum of gradient")
+    parser.add_argument("--b2", type=float, default=0.999, help="adam: decay of second order momentum of gradient")
+    parser.add_argument("--latent_dim", type=int, default=100, help="dimensionality of the latent space")
+    args = parser.parse_args()
+
+    main(args)

src/lightning/pytorch/strategies/ddp.py

@@ -214,7 +214,7 @@ def set_world_ranks(self) -> None:
         rank_zero_only.rank = utils_rank_zero_only.rank = self.global_rank
 
     def _register_ddp_hooks(self) -> None:
-        log.debug(f"{self.__class__.__name__}: registering ddp hooks")
+        log.debug(f"{self.__class__.__name__}: registering DDP hooks")
         # currently, DDP communication hooks only work with NCCL backend and SPSD (single process single device) mode
         # https://github.com/pytorch/pytorch/blob/v1.8.0/torch/nn/parallel/distributed.py#L1080-L1084
         if self.root_device.type == "cuda":
@@ -419,6 +419,39 @@ def teardown(self) -> None:
         super().teardown()
 
 
+class MultiModelDDPStrategy(DDPStrategy):
+    @override
+    def _setup_model(self, model: Module) -> Module:
+        device_ids = self.determine_ddp_device_ids()
+        log.debug(f"setting up DDP model with device ids: {device_ids}, kwargs: {self._ddp_kwargs}")
+        # https://pytorch.org/docs/stable/notes/cuda.html#id5
+        ctx = torch.cuda.stream(torch.cuda.Stream()) if device_ids is not None else nullcontext()
+        with ctx:
+            for name, module in model.named_children():
+                if isinstance(module, Module):
+                    ddp_module = DistributedDataParallel(module, device_ids=device_ids, **self._ddp_kwargs)
+                    setattr(model, name, ddp_module)
+            return model
+
+    @override
+    def _register_ddp_hooks(self) -> None:
+        log.debug(f"{self.__class__.__name__}: registering DDP hooks")
+        # currently, DDP communication hooks only work with NCCL backend and SPSD (single process single device) mode
+        # https://github.com/pytorch/pytorch/blob/v1.8.0/torch/nn/parallel/distributed.py#L1080-L1084
+        if self.root_device.type != "cuda":
+            return
+        assert isinstance(self.model, Module)
+
+        for name, module in self.model.named_children():
+            assert isinstance(module, DistributedDataParallel)
+            _register_ddp_comm_hook(
+                model=module,
+                ddp_comm_state=self._ddp_comm_state,
+                ddp_comm_hook=self._ddp_comm_hook,
+                ddp_comm_wrapper=self._ddp_comm_wrapper,
+            )
+
+
 class _DDPForwardRedirection(_ForwardRedirection):
     @override
     def on_after_inner_forward(self, wrapper_module: Module, original_module: "pl.LightningModule") -> None: